Loud-speaker



P. VARNET LOUD-SPEAKER Nov. 3, 1953.

Filed March 24, 1950 m Q azes #744 7 LL 5/ Z Awavrae RUE Patented Nov. 3, 1953 LOUD-SPEAKER Pierre Varnet, Romainville, France Application March 24, 1950, Serial No. 151,724

Claims priority, application France March 31, 1949 1 Claim. 1

Continued efforts are being made in the loudspeaker art in order to increase the volume output of loud-speakers on the one hand, and their tone quality or fidelity of reproduction on the other.

With the purpose of deriving maximum power efiiciency from a loud-speaker, it has been attempted to reduce the magnetic gap across which the movable coil of the loud-speaker is displaced. A limitation is of course soon provided in this respect by the actual thickness of the coil itself and considerations of electrical efliciency prevent any attempt to reduce the volume of wire used below a'definite value.

From the standpoint of fidelity, the chief obstacle lies in the plurality of resonance conditions possessed by the diaphragm owing to the fact that, while this element operates as a piston that moves bodily at low frequencies, it becomes the locus of radial wave propagation at intermediate frequencies and especially in the upper frequency ranges of the audible spectrum. Reflection of such waves on their reaching the corrugated outer margin by which the diaphragm is suspended creates standing waves and disturbing resonance conditions. It has been attempted in the past to overcome this objectionable effect by endeavouring to absorb in a gradual manner the propagated energy so that only a very small residual fraction thereof will remain as the Wave of energy reaches the peripheral area of reflection. The procedures used heretofore for this purpose may be considered as falling into one or more of the following classes:

Gradually decreasing the impregnation of the diaphragm from the centre to the periphery;

Use of a highly absorptive substance;

Use of concentrical corrugations adapted to reflect and absorb the energy in a progressive manner;

Use of specially-designed contours for the diaphragm (such as the so-called exponential contour) in which the nearly flat radially outer areas oppose transmission of the higher frequencies; and

Use of diaphragms that are not surfaces of revolution (e. g. elliptic surfaces) in order to obtain varying vibratory conditions in the different azimuths.

However, none of the above-noted measures eliminates the difliculty satisfactorily, in that they all result in the introduction of further defects individually inherent to them; thus, the first four above-mentioned methods emphasize a defect which is only too prone to occur of itself innermal conditions, namely that only the centralarea of the diaphragm effectively participates in radiating the higher-pitched notes; in the case of elliptic diaphragms the operation is disturbed at all frequencies because a flexible surface when uniformly loaded tends naturally to assume the form of a surface of revolution, and this results in the occurrence of secondary movements in the diaphragm, so that this solution is now rejected by experts.

Another well-known difficulty in sound reproduction consists in the directional operation of loud-speakers at the higher frequencies. it is known that the useful width of the radiated sound beam decreases sharply as the frequency exceeds a definite value, so that the tone quality of the sound heard deteriorates as the hearer deviates substantially from the centre axis of the loudspeaker. 'I'o attenuate this drawback, special loud-speakers have for instance been designed for the high-pitched sounds, or further the number of loud-speakers provided for a given area of space has been increased.

It -is an object of this invention to provide various improvements whereby very substantial advances towards improving both the volume and tone fidelity :of a luod-spea'ker are achieved, by the use of astonishingly simple and economical means.

An object-of the invention is to provide :a loudspeaker of high power efficiency, having a response characteristic free of any sensitive resonance peaks and satisfactorily reproducing the higher audible frequencies.

A more specific object is to provide a method of movable coil construction whereby the dimensiohs of the coil are markedly reduced without substantially affecting its efficiency and imparting to it increased strength and longer life.

A further object is to create on a loud-speaker diaphragm such conditions of wave propagation that reflection is rendered gradual both in time and space, and that resonance peaks are practically eliminated.

A further object is to increase in a loud-speaker the solid angle of acceptable reproduction for-the higher audible frequency ranges.

One characteristic feature of the invention is concerned with the moving coil: over the external layer of the winding of this coil th wire is stripped of insulation and is cut into or flatted, so as to present a substantially smooth outer cylindrical surface.

The invention provides a method of producing uch a type of coil, termed herein a scraped coil, which comprises, immediately after impregnation or after having wound the coil in the presence of an adhesive composition, and while the adhesive is still somewhat plastic, subjecting the coil to a scraping operation by means of an appropriately formed tool on a lathe, this scraping step on the lathe being controlled with the naked eye or preferably with a gauge in such a way as to remove an annular layer comprisiny the full external layer of insulation as well as a small depth of the outer metal of the outer turns of wire throughout the projecting areas thereof.

This leads to a very substantial saving in space within the magnetic gap, by the suppression of the useless outer layer of insulating coating plus a certain depth of metal, and the fact that during this operation the turns of wire become packed against one another in a body of adhesive still in its plastic state provides for a compensation for any inevitable irregularities as may result from irregularities in the wire itself, in the winding, and/or in a lack of homogeneity in the adhesive used. As a result it becomes superfluous to provide for any special tolerances in order to make allowance for the external dimensional irregularities of the coil. The tools action at the same time smoothes out the internal stresses, eliminating or reducing in particular the usual tendency of the last turn of the coil winding to spring out and project above the surface of the coil.

Another feature of the invention lies in the fact that a number of radial areas of heterogeneity are created over the loud-speaker diaphragm, giving rise to propagatory conditions differing from those prevailing throughout the remainder of the diaphragm surface. The diaphragm retains a general form of a surface of revolution, but according to the invention azimuthal heterogeneity is created resulting in variations in the velocity of propagation along the azimuth or azimuths under consideration. Under these circumstances, instead of there being as in known diaphragms a sharp and total reflection of the wave at the periphery of the diaphragm, the energy in motion from the centre towards the periphery of the diaphragm is gradually reflected. As a result, while the full surface of the diaphragm remains effectively employed in radiating frequencies even in the highest audible ranges, stray resonance is practically eliminated in a diaphragm according to the invention. It is suggested that such a diaphragm be termed "progressively-refiective azimuthally heterogeneous diaphragrns.

Tests performed with the loud-speaker to be described by way of example hereinafter have corroborated the above theoretical forecasts in that they showed a marked flattening out of the response characteristic and total disappearance of disturbance noises due to the resonance occurring in a smooth conical diaphragm, without any detriment to the width of the range of reproduced frequencies.

Azimuthal heterogeneity according to the invention may for instance be obtained by differential im regnation of the areas that are to have differential velocities of wave pro agation, or alternatively by the use of corrugations of appropriate contours, or both.

Another aspect of the operation of azimuthally-heterogeneous diaphragms according to the invention resides in the fact that during the propagation of the wave over the diaphragm,

the wave front assumes an increasingly great inclination relatively to the axis of the diaphragm. Consequently, radiation at high audible frequencies is not only effected in the direction of said axis, but also along directions forming with said axis an angle that is the greater as the heterogeneity in wave velocity is more marked, and therefore also, as the wave front is more inclined as the wave moves away from the centre of the diaphragm. In other words, the solid angle of radiation of the higher audible frequencies is increased.

The objects, features and advantages of the invention will become more fully apparent from the ensuing description relating to one embodiment thereof selected merely by way of example, with reference to the accompanying drawings in which:

Figure 1 is a front view of a loud-speaker according to the invention;

Figure 2 is a corresponding axial cross-section;

Figures 3 and 4 are detail views in cross-section of a moving coil according to the inven tion, respectively before and after the scraping step; and

Figure 5 is an explanatory diagram illustrating one way of performing the scraping operation according to the invention.

In the exemplary embodiment of the invention illustrated, a loud-speaker according to the invention comprises a diaphragm H mounted on a frame 12 and carrying centrally thereof a movable coil of the above-defined scraped type, produced as more specifically described hereinafter. This coil is arranged in a manner known per se in the magnetic gap of an electro-magnet it the field winding of which is visible at I5. In place of an electro-magnet a permanent magnet could of course be used. The central part of the diaphragm is suspended from the core (5 of the magnet in known manner through a suspension member or spider I1.

The diaphragm is in the general form of a circular cone and is secured at its radially outer periphery in the conventional way through a flexible part I 8, and is formed with a set of undulations such as i9, 213, 2! each of which, as shown, has a maximum width and depth at its intersection with one radius A of the diaphragm, gradually fading away on each side of this radius so as to be practically nonexistent on reaching the radius 13 diametrically opposed to A. It is quite simple in practice to provide undulations of such form. It is merely necessary for this purpose when machining the undulations, to place the mould out-of-centre with respect to the mandrel of the lathe.

Moreover, considering the successive undulations of increasing radius from the centre to the periphery of the diaphragm, it will be seen that their plane is more and more steeply inclined with respect to the axis of the cone. To achieve such increasing slope, it is only necessary to suitably displace the part on the mandrel of the lathe and to proceed turning the part with an increasing eccentricity.

Owing to this arrangement, the velocity of propagation is a minimum along the radius A and a maximum over the radius B, and there is a gradual and smooth progression from one to the other of these conditions. In the specific construction illustrated, the velocity of propagation over the radius A is equal to three fourths the velocity over the radius B. Consequently the wave front propagated from the centre to the periphery of the diaphragm will assume an increasing inclination with respect to the axis of the loud-speaker and the inclination of the undulations is so selected as to match this progression, so that the wave front will attain all the points of a given undulation at the same instant. This will prevent mutual interference between the velocities of propagation.

Experience confirms the fact that the peculiar distribution of reflected waves in time and space thus achieved provides an extremely regular response characteristic for the loud-speaker while retaining the full surface of the diaphragm for transmission of even the highest frequencies and thus in no way impairing the extent of the reproduced frequency spectrum.

The number of axes of heterogeneity such as AB, equal to unit in the exemplary embodiment just described, may be varied in accordance with the specific characteristics desired in each instance, and the nature and dimensions of the diaphragm, but will generally lie between 1 and 8. Moreover, the various areas of heterogeneity need not be identical; the greater their number, the more difficult will it of course be to achieve substantially different speeds of propagation.

In the case of a diaphragm having more than one area of heterogeneity, the plane of the undulations will also be gradually made steeper as the radius of the undulation under consideration increases so that in this case also the wave front will reach all the points of any given undulation simultaneously or substantially so. In other words, the undulations are so formed that their general shape in cross-section on a radial plane will be similar to the general form of the wave front as it passes through the undulation under consideration.

Instead oi using undulations having the particular contour illustrated to provide the desired heterogeneous character in the diaphragm surface which is a feature of the invention, it would also lie within the scope of the invention to use the method consisting of diirerentially impregnating various areas or the diaphragm to achieve an equival nt efiect. In this case, the first step may be to subject the diaphragm as a whole to 'a moderate impregnation with a varnish the chief function of which is to render the diaphragm impervious to moisture and then additional impregnation is made using an appropriate varnish applied to the radial zones of the diaphragm in which it is desired to create an increased propagational velocity. During this step, a mask may be used for protecting from the varnish all but the radial Zones selected; a similar result may alternatively be achieved by using a set of suitably-arranged spray-guns.

This method involving heterogeneous impregnation may be used alone or in combination with the dissymmetrical undulations described above.

It is essential to observe that in neither of the two alternative methods described above for cre- Y ating dissymmetrical radial zones of wave propagation in the diaphragm, is the tangential elasticity of the diaphragm in any way affected, as this would have the drawback of impairing the propagation of the higher-frequency vibrations.

Various other methods of imparting the desired angular heterogeneity to the diaphragm may be devised within the scope of the invention. Thus, I may mention as a further example, the procedure consisting of creating the radial areas of increased wave velocity by bonding to a base diaphragm sector-shaped elements provided with ribs or corrugations in the radial direction, which will locally increase the moment of inertia and thus create an additional mode of propagation in the area under consideration.

I It should be noted in thi connection that throughout the specification and claims, the term rib or corrugation is used to designate a fold or flute formed in a radial direction, in contradi'stinction to what herein is termed an undulation i. e. a corrugation defined by the intersection with the diaphragm of planes variably inclined with respect to the axis of the diaphragm.

Ihe undulations act upon the radial elasticity, and the varnish affects both the radial and'transverse elasticity; ribs simply embossed on a diaphragm would increase its tangential elasticity; but if, as suggested above, a ribbed element is bonded to a diaphragm base surface, then the tangential stiffness will be retained in full.

Figures 3 and 4; relate to the movable coil 'ac cording to the invention. In Figure 3, there is illustrated a movable coil with two layers of wire such as it may appear while still on its winding mandrel and in its heated and as yet non-set condition from the impregnation step, or after having been wound in the presence of adhesive, some of the various defects such a coil may offer being purposely exaggerated in the drawing; thus represents a foreign body and 26 an aggregate of adhesive, both acting to produce surface irregularities or bulges over the theoretically cylindrical surface of the mandrel or core 21 and a corresponding displacement of the turns 28 and 29 of the first course of the winding. 30 represents another aggregate of adhesive which raises the turns 31 and 32 of the second layer or course. The turn 33 is shown as possessing increased thickness due to a defect in the coating operation; and 34 is the last turn of the coil and accordingly tends to spring out or escape due to its resiliency. The over-all thickness dimension of the movable coil is that indicated by the dimension H. Thus an examination of this figure will explain why it is necessary to keep comparatively large dimensional tolerances in the magnetic gap in excess of the theoretical thickness of the movable coil.

According to the invention, the coil is subjected, in its heated condition as it is withdrawn from its impregnating bath or alternatively in its as yet non-set condition if it has been wound in the presence of adhesive, to a turning operation on a lathe with an appropriate tool to remove the entire external depth of insulation, plus .a certain depth of the outer metal. This scraping action should be effected in the direction of winding, so as to terminate at the outlet of the coil. Owing to the radial pressure of the tool, the turns of the Winding are tightly packed against one another. The more outer turns will then assume a'circular shape without any elastic tension and will no longer tend to escape on the coil being stripped from its mold. Any defects or irregularities that may have occurred in the manufacture of the wire or in the winding operation are removed or considerably reduced,the compression of the turns being facilitatedbecause the impregnating composition and the insulating coating of the coil are both still hot and plastic during this operation.

Thus it will be seen as in Figure e that the foreign body 2:; and the aggregates 26 and '21 have been squashed or squeezed out into the interstices between the turns of wire by the action of the scraping tool, correspondingly improving the regularity of the winding. The turns 28, 29, 31 and 32 have moved in closer to their proper positions in the winding; the turn 33 is simply cut into a little more deeply than the other turns. Where the Winding contains foreign bodies of such a nature as to withstand the pressure from the tool, the desired regularity is achieved by cutting into the wire.

The ultimate turn of Wire 34 has fallen in into line with the other turns and no longer shows any tendency to spring away, This is because, as shown by tests, the scraping action of the tool has subjected the external layer of metal to a packing action which tends to cause the wire to conform with the general form of the winding and eliminates any tendency of the turns, and especially the ultimate turn, to open out.

As a result, the thickness dimension of the coil is now that indicated at h; the saving in space thus accomplished is self-evident and chiefly corresponds to the elimination of the dimensional tolerance or clearance required, prior to or in the absence of, the scraping step according to the invention, by the irregularities mentioned above.

In performing the scraping step, there are optimum conditions of temperature and/or degree of set which may easily be ascertained by test for any given type of varnish composition; this temperature should be high enough for the impregnating composition still to retain a certain degree of plasticity, while being sufficiently low to cause the coat of insulation varnish on the wire to be hard enough to resist crushing under the pressure of the tool, in order to avoid the danger of short circuiting the turns of wire at any point.

Actually the scraping operation is extremely easy to carry out and does not require complicated equipment for its performance. It may indeed be effected by hand, the movable coil being retained on the core which served for the winding operation. Thus, the procedure illustrated in the diagram of Figure may be used, the movable coil on its winding core being shown at 35, and the tool used being shown as simply comprising a shank into one end of which is inserted a wedge-shaped tool El blocked with a setscrew the tool being held in the operators hand. The coil is set into rotation in the direction of the arrow, and the tool is then simply moved into engagement with the surface of the coil and held against it with a slight degree of manual pressure until the coil surface assumes a typical glossy watered or moire aspect denoting that the outer layer of enamel and adhesive have been removed to a suflicient depth. This operation only requires a few seconds. The base of the shank 36 may first be rested as at 33 onto any convenient support, so that the tool will initially be presented in a position such as 317, then the shank may be gradually shifted to the position shown in chain lines with the tool at 31:! so that the cutting angle assumes a favorable value, and the desired result is instantaneously obtained.

By way of indication, in the practical instance of a coil comprising two layers of enamel-coated wire 51 mm. in diameter, the following results have been obtained. Removal of the outer coating of enamel yields a saving of mm.; the wire itself was cut into to a depth of mm.; finally, the packing of the turns and consequent elimination of the tolerances indispensable in the conventional methods of coil manufacture resulted in a reduction of mm. on the thickness of the coil. Thus an over-all saving of mm. was secured, or over 10% in the case of the usual magnetic gap of about mm., which it was normally necessary to provide in the case of a coil of the type under consideration, and which it thus became possible to reduce in a corresponding degree.

This of course entails a proportionate increase in the magnetic field and the efiiciency factor of the loudspeaker, all other factors remaining equal.

It may be observed in particular that a given saving in thickness as obtained by removing the external segments of the cross-section of the outer turns of wire, reduces only in a comparatively very low proportion the conductance of the winding, whereas the increase in magnetic field made possible by the corresponding reduction in the magnetic gap is directly proportional, in practice, to this saving in thickness, so that in line the balance of the operation shows an ample profit. Moreover, the coil is lightened by the amount of material removed.

It may further be stated that a coil scraped as described acquires a highly compact and strong structure owing to the packing of the turns of winding, which from place to place has the effect of filling up any voids by adhesive composition squeezed out from areas where the adhesive was in excess.

Another advantage highly desirable in quantity production is that in the event the winding had received an excess turn during the winding operation, this excess turn will be necessarily cut off in the scraping step and the winding will be automatically rejected, thereby eliminating an operation of production control.

It should of course be understood that the invention is not restricted to the specific forms of embodiment illustrated and described by way of example. It will be clear that many modifications in and combinations of the steps of procedure described, and various combinations thereof with known ways and means may easily be conceived by those familiar with the art without exceeding the scope of this invention.

Thus in particular, the method of imparting heterogeneity to the loud-speaker diaphragm according to the invention, is applicable concurrently with one or more procedures known in the art, in the case of diaphragms of any special contour, and of elliptical diaphragms. The latter instance may give rise to a particularly advantageous application; namely, the heterogeneous structure involving one radial zone of heterogeneity may be applied to an elliptical diaphragm with the direction of maximum velocity extending along one of the minor axes of the ellipse, in such a way as to obtain wave fronts approaching circumferential shape and thus improving the operation of the elliptic diaphragm, especially from the standpoint of reproduction of the higher audible frequencies and of transient vibratory conditions.

What I claim:

An acoustical diaphragm in the general form of a conical surface of revolution formed of material of substantially uniform thickness, said conical diaphragm being provided with means driving it at the center thereof, said conical diaphragm having a plurality of arcuate undulations extending substantially circumferentially about at least a portion of the circumference thereof and varying progressively in width and depth circumferentially of the conical diaphragm from a maximum along one radius to a minimum along a radius spaced 180 from the first mentioned radius, said undulations lying in planes with the planes of the undulations being substantially perpendicular to the axis of the conical diaphragm for the undulations nearest the center of the conical diaphragm and being inclined more and more to said axis as the distance between the undulations and the center of the conical diaphragm increases.

PIERRE VARNET.

References Cited in the file of this patent UNITED STATES PATENTS Number Number 15 Number Name Date Ellis et a1 Oct. 8, 1912 Nicolson -1 Apr. 12, 1927 Lane July 39, 1929 Tennessee et a1 Got. 5, 1931 Bedford Feb. 23, 1932 Jensen Jan. 2, 1934 Engholm May 1, 1934 Cunningham Feb. 11, 1936 Pare July 7, 1942 Gersch June 20, 1950 Lawrance Nov. 28, 1950 FOREIGN PATENTS Country Date France Dec. 26, 1932 

